Handle -stdlib++-isystem in cargo build script path rewriting

The -stdlib++-isystem flag is a standard Clang flag for specifying
C++ standard library header paths. When Bazel passes CC toolchain
flags to cargo build scripts, paths following this flag need to be
prefixed with ${pwd} just like -isystem paths, otherwise the build
script can't find the headers in the sandbox.

Author: Tamas Vajk

cargo/private/cargo_build_script.bzl

@@ -249,7 +249,7 @@ def _pwd_flags_fsanitize_ignorelist(args):
 
 def _pwd_flags_isystem(args):
     """Prefix execroot-relative paths in -isystem and -Xclang -internal-isystem arguments with ${pwd}."""
-    return _prefix_pwd_to_flag(args, ["-isystem", "-internal-isystem"])
+    return _prefix_pwd_to_flag(args, ["-isystem", "-internal-isystem", "-stdlib++-isystem"])
 
 def _pwd_flags_L(args):
     """Prefix execroot-relative paths in -L arguments with ${pwd}."""